According to the letter, the House Energy and Commerce Committee has been looking into "negotiations and agreements made between representatives from the White House Office of Health Reform (WHOHR) and various health care industry stakeholders regarding health care reform legislation" for over two years now. Republicans on the Committee say that the White House has refused to cooperate by turning over internal memos. But they also say that those memos made their way to Lizza, and that the White House has now waived any assertion of executive privilege:

Finally, while the White House has so far studiously avoided asserting executive privilege and has simply refused to provide the requested information, by voluntarily providing this information to a reporter the White House has waived any right to refuse production of these materials based on claims of privilege.

A footnote to this sentence reads:

In In Re Sealed Case the D.C. Court of Appeals addressed this issue when it held the White House had waived claims of privilege in regards to documents it voluntarily revealed to third parties outside the White House.

The Republicans' request is appropriately tailored to the documents they seek (and claim have been voluntarily released) under the approach to the executive privilege and the deliberative privilege In Re Sealed Case, but there's no indication that the White House voluntarily released them to Lizza--that Lizza didn't get them from some other source, or that the White House provided only selected information from them (and not the memos themselves). Absent a voluntary release of the memos themselves, the White House almost certainly has not waived any available privilege. And In Re Sealed Case suggests that the White House is under no obligation to assert the privilege until the Committee seeks to compel release (although In Re Sealed Case dealt with a different situation--an assertion of privilege against the OIC, not a congressional committee).